1. Field of the Invention
The present invention relates to a thermally conductive compound which is suitable as heat transfer means which rapidly acts to transfer heat generated by the operation of a semiconductor integrated circuit element to a radiation part as well as a semiconductor device using the same.
2. Description of the Related Art
A semiconductor element, for example, semiconductor integrated circuit element, is tightly sealed up, for example, in a semiconductor element housing package to avoid contact with the air when used, and the resulting semiconductor device is connected electrically and mechanically to an outer electric circuit to use.
The semiconductor element housing package generally consists of an insulator body, in which a conductor wiring is formed in a prescribed pattern and on the surface of which a portion for mounting a semiconductor integrated circuit element (element mounting portion) is provided, and a cap which covers the element mounting portion and forms a hollow cut off from the outside between the body and a circumference provided at the part surrounding the element mounting portion on the upper surface of the body. The semiconductor integrated circuit element is then placed in the element mounting portion of the semiconductor element housing package, and then the cap is closely placed on a substrate surrounding the element mounting portion and adhered thereto with a heat curing resin, etc., to form a cell consisting of the insulator body and the cap. The semiconductor integrated circuit element is placed inside the tightly sealed hollow of the cell to make a semiconductor device.
In recent years, high integration of semiconductor integrated circuit elements considerably increases and there is a tendency to increase markedly the heating value during the operation of the semiconductor element, so it becomes an important problem to rapidly release heat generated by the semiconductor integrated circuits outside of the semiconductor device in order to prevent abnormal operation of the element arising from overheating.
In order to avoid this problem, a thermally conductive compound having high thermal conductivity is inserted between the upper surface of a semiconductor integrated circuit and the cap in the airtight sealed-up portion to make a practical structure, and a highly heat conductive material such as copper or aluminum is used as the cap material, or as an alternative method, a material for heat dissipation such as a radiation fin on the upper surface of the cap is provided so that heat generated by the semiconductor integrated circuit element is rapidly transferred to the cap, from which the heat was rapidly released outside.
When a cap made of copper or aluminum is used, however, a shearing stress is repeatedly added to the thermally conductive compound inserted between the semiconductor integrated circuit element and the cap of which the expansion/shrinkage rate is different, by the repeated rise and drop in temperature accompanied by the repeated work and rest of the semiconductor integrated circuit element, because the thermal expansion rate is greatly different between the cap and the semiconductor integrated circuit element mainly made of a silicon material (linear thermal expansion rate at 40.degree. C. is 1.678.times.10.sup.-5 /deg for copper, 2.313.times.10.sup.-5 /deg for aluminum, and 2.4.times.10.sup.-6 /deg for silicon) and in addition the thermally conductive compound is used in a state closely adhesive and chemically or physically linked to the cap and the semiconductor integrated circuit element in order to enhance the thermal conductivity at the surfaces of the thermally conductive compound with the cap and with the semiconductor integrated circuit element. As a result, it is afraid that the thermally conductive compound might be destroyed or the semiconductor integrated circuit might be destroyed by a reaction, or cracks or peeling-off might be produced at the surface of the thermally conductive compound with the circuit element or cap. Accordingly, a variety of thermally conductive compounds and semiconductor devices have been proposed because the merely high thermal conductivity is insufficient.
For example, Japanese Unexamined Patent Publication JP-A 61-29162 (1986) has proposed a semiconductor device that keeps in contact with a thermally conductive material, in which at least one of the upper surface of a pellet (semiconductor integrated circuit element) and the back surface of a cap or the back surface of the pellet and the upper surface of the substrate, to which the pellet is fixed, comprises silicon gel interposed between them, or silicon gel containing a filler consisting of alumina or silicon carbide. According to this device, by making a thermally conductive material comprising silicon gel interposed between the upper surface of the pellet and the backside of the cap, heat generated in the pellet can be released to the cap to effectively prevent overheat of the pellet. Moreover, since the silicon gel has elasticity, mechanical destruction such as occurrence of cracks on the pellet by temperature change can be avoided. Further it is also possible to release the heat more effectively by adding a filler comprising alumina or silicon carbide as a thermally conductive material with high thermal conductivity.
In addition, Japanese Unexamined Patent Publication JP-A 61-36961 (1986) proposes a multi-chip integrated circuit package which comprises a circuit substrate, a plural number of integrated circuit chips (semiconductor element) which are connected electrically or mechanically to the circuit substrate through a plural number of flexible leads provided in advance on the body, a plural number of heat radiating plate fixed to a plural number of integrated circuit chips in one-to-one correspondence through the first thermally conductive adhesive, a heat radiation cover which covers the chip-mounted surface and is fixed to a plural number of heat radiating plates through the second thermally conductive adhesive, and a heat sink fixed on the heat radiation cover. As for the thermally conductive adhesives, an epoxy adhesive containing a highly thermally conductive silver-filler and tin-lead eutectic solder are disclosed. According to this multi-chip integrated circuit package, heat generated by the integrated circuit chips is effectively transferred to the heat sink to effectively cool the chips, and the improvement of radiation characteristics can be attained.
In addition, Japanese Examined Patent Publication JP-B2 6-95557 (1994) has proposed a thermally conductive compound which comprises a liquid carrier selected from the group consisting of mineral oils and poly(.alpha.-olefin) oil, thermally conductive filler particles which are dispersed in the liquid carrier so as to wet the particle surface, and a binder which has at least one functional group reacting with the surface of the thermally conductive filler particles and at least one functional group having the same polarity as the liquid carrier, the binder being selected from the group consisting of organosilanes, organotitanates, organoaluminates and organozirconates. According to the patent publication, the thermally conductive compound is a phase-stable and thermally conductive compound that contains a coupling agent (binder) selected from the group consisting of organometals which has at least a dielectric liquid carrier, highly thermally conductive fillers (used for filling) homogeneously dispersed in the carrier, and a functional group reacting with the particle surface and another functional group preferentially moistening the particles by self-coagulation. The compound is adaptable to any chip surface of which the roughness, slope and height are different, is able to moisten the surface, exhibits high filling density, maximizes the thermal conductivity, and maintains a homogeneous suspension to prevent phase separation. As a result, a thermally conductive compound that hardly causes phase separation, though the viscosity is low, sparingly flows out, and is suitable for the application to integrated circuit chips, can be provided.
Moreover, a treatise by Lacombe et al., "FLEXIBLE HEAT-CONDUCTING SHEET MATERIAL FOR SEMICONDUCTOR PACKAGES" published in IBM Technical Disclosure Bulletin, April 1983, vol. 25, no. 11A, 5740-5743, has proposed a thermally conductive compound which is prepared by filling ceramic particles consisting of boron nitride of 1-15 .mu.m in diameter into a polymer carrier consisting of polyisobutylene. The compound, according to the disclosure, is subjected to a step of being placed as a flexible sheet material on the chip mounted on the multi-layer ceramic substrate, or is laminated as sheets in the inside of an aluminum cap to use as means for transferring heat from a chip to a cap. According to the disclosure, when the molecular weight of polyisobutylene is made higher than 100,000, the content of chemically inactive boron nitride particles with high thermal conductivity and low electric conductivity in the polymer carrier can be made 90 weight % or higher. Since polyisobutylene which is a viscoelastic liquid becomes rubbery at room temperature at the high molecule, the cap can be disposed by making the thermally conductive material mediate in one step as mentioned above, and labor saving can be attained much more than in the usual grease-supply method.
The respective prior arts as mentioned above, however, had the following problems.
The semiconductor device as proposed in JP-A 61-29162 (1986) has such a problem that the silicon gel as a thermally conductive material, which usually has a low thermal conductivity of approximately 0.2-0.3 W/mK, insufficiently transfers heat to probably overheat the semiconductor integrated circuit elements when used as heat transfer means for the current semiconductor integrated circuit elements of which the calorific value strikingly increases. Moreover, it was also problem that since the viscosity of silicon gel is high, when the filler is intended to charge at a high rate, there is formed cracks in the filler because of insufficient distribution of the silicon gel so that the thermal conductivity of the thermal conductive material cannot be increased sufficiently.
On the other hand, in the integrated circuit package proposed in Japanese Unexamined Patent Publication JP-A 61-36961 (1986), it was problem that since the integrated circuit chip and the radiation plate are adhered with a thermally conductive adhesive, when the coefficient of thermal expansion is considerably different between both materials, mechanical stress due to the temperature difference of both materials is exerted on the integrated circuit chips possibly causing destruction, so the coefficient of thermal expansion of the radiation plate is required to be equal to that of the integrated circuit chips, the choices of the material of radiation plate is limited, and the radiation plate cannot be produced with a material having high thermal conductivity.
In Japanese Unexamined Patent Publication no. 61-36961, an example of a TAB type of integrated circuit chips which have a plural number of lead wires of tape type is shown, wherein bending formation is applied to the circuit chip and then the chip is lead-bonded on the substrate, so that no cut of the lead wire occur by vertical or horizontal movement of the chip body, and accordingly, as described, the quality of the radiation cover material can be determined in primary consideration of the thermal conductivity but disregarding the coefficient of thermal expansion. It was a problem, however, that when a method for chip disposition other than the TAB method is employed, for example, when the chip is mounted on the substrate through a solder layer by a flip-chip mounting, the use of this radiation structure causes destruction of the solder layer with vertical or horizontal movement of the chip body and the connection of the integrated circuit with the substrate cannot be maintained.
On the other hand, the thermally conductive compounds proposed in Japanese Examined Patent Publication JP-B2 6-95557 (1994) are grease-like thermally conductive compounds prepared by dispersing thermally conductive filler particles in an organic liquid as a carrier, and when the compounds are used as thermally conductive materials in a semiconductor device, moisture contained in an airtight sealed-up cavity sometimes acts on the binder contained in the thermally conductive compounds to join together; as a result, there is a fear that the capacity of the thermally conductive filler particles becomes insufficient and cause phase separation between the organic liquid as carrier and the thermally conductive filler particles. As a result of the phase separation, the organic liquid flows out to cause shortage of amount and to form cavities of flowing-out, which disturb heat transfer or produce aggregate of the thermally conductive filler particles making the heat transfer uneven in the thermally conductive compounds or causing decrease of mechanical strength of the thermally conductive filler particles such as falling-off of the thermally conductive filler particles.
In the thermally conductive compounds proposed in a report of IBM Technical Disclosure Bulletin, the polyisobutylene as a polymer carrier, when the heat cycle test (the reliability test for confirmation whether there is no trouble in a semiconductor device in the appearance or specific quality which might be produced by the repetitive temperature change caused by environment where the device is used or by generation of heat from the semiconductor integrated circuit element, usually repeated 50-1000 cycles at a certain cycle at lower temperatures of -50 to -40.degree. C. and at higher temperatures of +125 to 150.degree. C.) which is required to assure the reliability of a semiconductor device is applied thereto, loses the plasticity at lower temperatures to become hard, so it was a problem that the thermally conductive compounds cannot relieve thermal stress generated by the difference of coefficients of thermal expansion between the chip and the cap to cause destruction inside of the thermally conductive compound or around the contacting surface of the chip of the thermally conductive compound or to peel off from the cap surface because the compound cannot stick thereto consecutively.